top_hits aggregation: Fail with a parse exception if `top_hits` agg is defined under a `nested` agg.

PR for #8668
But then it should work right? For instance if you first have the following aggregation tree:
 - level 1: nested aggregation
 - level 2: buckets aggregation based on some nested properties
 - level 3: reverse_nested aggregation to go back to the root docs
 - level 4: top_hits aggregation to get the top hits per bucket

Then the `top_hits` aggregation is legal since it applies to root documents although it has a parent `nested` aggregation?